Summary

The most worrisome of the avian flu viruses to emerge in recent years is looking even more menacing. Since it first began killing people in eastern China 2 years ago, the H7N9 virus has infected poultry throughout the country and could be poised to spill into Central Asia. Worse, strains of H7N9 are promiscuously swapping DNA with other avian viruses in circulation, a team reports in Nature this week. Public health officials are walking a fine line: heightening vigilance for any signs of H7N9 evolving into a form that could spread rapidly among people, while preaching caution that it or other bird flu viruses are not, well, about to go viral.